Stabilized carbon cluster conducting or superconducting material, process for producing the same, device using carbon cluster, and process for producing the device

ABSTRACT

A stabilized carbon cluster conducting material comprising (i) a core comprising a conducting or superconducting carbon cluster and (ii) a sheath covering the core; a device comprising a substrate having thereon a film of a conducting or superconducting carbon cluster covered with a protective film capable of substantially preventing permeation of oxygen and water in the atmosphere; and processes for producing the stabilized carbon cluster conducting material and the device.

This is a Continuation of application Ser. No. 08/051,345 filed Apr. 23,1993, now abandoned.

FIELD OF THE INVENTION

The present invention relates to a stabilized carbon cluster conductingor superconducting material, a process for producing the same, a deviceusing a carbon cluster conducting material, and a process for producingthe device.

BACKGROUND OF THE INVENTION

It is known that a powder, thin film, etc. of carbon cluster moleculeseach composed of a plurality of carbon atoms linked to form a sphericalshape or an ellipsoidal shape having a so-called fullerrenes structure,such as C₆₀ or C₇₀, is doped with an impurity atom, such as an alkalimetal, to provide an organic conducting material or a superconductingmaterial.

For example, a potassium-doped C₆₀ film and a rubidium-doped C₆₀ filmhave been reported to have conductivity of 500 S/cm and 100 S/cm,respectively (as described in Nature, Vol. 350, p. 320 (Mar. 28, 1991)).

It was also reported that potassium-doped C₆₀, K_(x) C₆₀, showssuperconductivity at a critical temperature (hereinafter abbreviated as"Tc") of 18 K. from the results of microwave absorption andsusceptibility measurements and at a Tc of 16 K. from the results ofresistivity measurement (as described in Nature, Vol. 350, p. 600 (Apr.18, 1991)) and that rubidium-doped C₆₀, Rb_(x) C₆₀, showssuperconductivity at a Tc of 28 K. (as described in Physical ReviewLetters, Vol. 66, p. 2830 (1991). Cesium-doped C₆₀, Cs_(x) C₆₀, wasreported to show superconductivity at a Tc of 30 K., and cesium- andrubidium-doped C₆₀, Cs₂ RbC₆₀, was reported to show superconductivity ata Tc of 33 K. (as described in Nature, Vol. 352, pp. 222-223 (Jul., 18,1991)).

Superconductivity of carbon clusters doped with Ca, an alkaline earthmetal, or Sn, a group IVb metal, was also reported. That is, Nature,Vol. 355, p. 529 (Feb., 6, 1992) reported superconductivity of Ca₅ C₆₀at a Tc of 8.4 K., and Solid State Commn., Vol. 82, No. 3, p. 167 (1992)reported superconductivity of a tin-doped C₆₀ /C₇₀ mixture, Sn_(x) C₆₀/C₇₀, at a Tc of 37 K.

Further, the present inventors have ascertained that a carbon clusterdoped with indium (In), a group IIIb element, can easily be obtained bysimultaneous vacuum evaporation of indium and a carbon cluster by meansof a vacuum deposition apparatus, an electron beam epitaxy (MBE)deposition apparatus, etc. and that the amount of the dopant In can becontrolled simply by varying the rate of In deposition in thesimultaneous vacuum evaporation so that fullerite conducting materialshaving an arbitrary conductivity, inclusive of from a nearly insulatorto a semiconductor-like material, can be prepared with extreme ease andgood reproducibility.

However, as has been well known, since an alkali metal element is highlyreactive and instable, when a carbon cluster doped with an alkali metalelement, e.g., K, Rb or Cs, is exposed to the atmosphere, the dopantelement undergoes reaction with oxygen or water in the atmosphere andthereby escapes from the carbon cluster, resulting in disappearance ofthe above-mentioned characteristics in a short period of time.

This tendency is observed with a carbon cluster doped with other metalelements. For example, the present inventors have revealed that a powderof C₆₀ doped with Ca, an alkaline earth metal, loses itssuperconductivity when left in air for 1 hour. According to theliterature cited above, the Sn-doped C₆₀ powder retains itssuperconductivity when allowed to stand in air for one day, butstability over a longer time remains unclear. The present inventors haveadditionally revealed that an In-doped C₆₀ film also loses itscharacteristics in air within a short time.

A currently widespread means for prevention of deterioration of metalelement-doped carbon clusters is to seal them in an inert gasatmosphere. For example, the doped cluster is put in a glass tubecontaining an inert gas, and the open end of the tube is heat sealed orsealed with an epoxy resin.

However, where the open end of a glass tube is heat sealed, the carboncluster is likely to undergo deterioration by the heat of the heatsealing. Where sealing is conducted with an epoxy resin, the resinpossibly undergoes cracking by changes in environmental temperature dueto the difference in thermal expansion coefficient between glass and theresin. If once a crack occurs, oxygen or water in air is no moreprevented from entering the glass tube, and deterioration ofcharacteristics cannot be avoided.

Besides, a carbon cluster once sealed into a glass tube as describedabove is difficult to be integrated into a device because of thedifficulty in connecting to outer wiring.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a stabilized carboncluster conducting or superconducting material which has excellentchemical stability and does not readily undergo a reduction inconductivity in air.

Another object of the present invention is to provide a stabilizedcarbon cluster conducting or superconducting material which retains itscharacteristics for an extended period of time even when used in theatmosphere as a wire or a cable and to provide a process for producingthe same.

A further object of the present invention is to provide a device using acarbon cluster conducting or superconducting film which is preventedfrom deterioration and thereby retains satisfactory conducting orsuperconductivity for an extended period of time and to provide aprocess for producing such a device.

Other objects and effects of the present invention will be apparent fromthe following description.

The present invention relates to a stabilized carbon cluster conductingmaterial comprising (i) a core comprising a conducting orsuperconducting carbon cluster and (ii) a sheath covering the core.

The present invention also relates to a process for producing astabilized carbon cluster conducting material comprising the steps of:filling a sheath with a mixture comprising a carbon cluster and adopant; and subjecting the sheath filled with the mixture to a dopingtreatment.

The present invention further relates to a device comprising a substratehaving thereon a film of a conducting or superconducting carbon clustercovered with a protective film capable of substantially preventingpermeation of oxygen and water in the atmosphere.

The present invention still further relates to a process for producing adevice comprising a substrate having thereon a film of a conducting orsuperconducting carbon cluster covered with a protective film, theprocess comprising the steps of:

depositing a carbon cluster on a substrate while adding a dopant to forma carbon cluster conducting or superconducting film on a substrate; and

forming a protective film impermeable to oxygen and water in theatmosphere to cover the carbon cluster conducting or superconductingfilm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 5 each illustrates a section of a stabilized carbon clusterconducting or superconducting material according to one embodiment ofthe present invention.

FIG. 6 illustrates a cross section of a device using the carbon clustersuperconducting film according to one embodiment of the presentinvention.

FIG. 7 illustrates a cross section of a device using the carbon clustersuperconducting film according to one embodiment of the presentinvention.

FIG. 8 is a schematic illustration of an MBE deposition apparatus whichcan be used for the production of the device according to the presentinvention.

FIGS. 9-(a), (b) and (c) illustrate the steps for producing the deviceof FIG. 7.

FIG. 10 illustrates a cross section of a Josephson junction device usingthe carbon cluster superconducting film according to the presentinvention.

FIGS. 11-(a), (b) and (c) illustrate the steps for producing the deviceof FIG. 9.

FIG. 12 is a graph of resistivity (Ω) vs. time of the device obtained inExample 5.

FIG. 13 is a composition profile of the device obtained in Example 5 inits thickness direction as measured by Auger electron spectroscopy.

FIG. 14 is a graph of resistivity (Ω) vs. temperature of the deviceobtained in Example 5.

FIG. 15 illustrates a cross section of a device produced in Example 6.

FIG. 16 is a graph of resistivity (Ω) vs. temperature of the deviceobtained in Example 6 before exposure to the atmosphere.

FIG. 17 is a graph of resistivity (Ω) vs. temperature of the deviceobtained in Example 6 after exposure to the atmosphere.

FIG. 18 illustrates a cross section of a device produced in Example 7.

FIG. 19 is a graph of resistivity (Ω) vs. temperature of the deviceobtained in Example 7 before exposure to the atmosphere.

FIG. 20 is a graph of resistivity (Ω) vs. temperature of the deviceobtained in Example 7 after exposure to the atmosphere.

DETAILED DESCRIPTION OF THE INVENTION

The carbon cluster which can be used in the present invention isrepresented by C_(2n), wherein 10≦n≦100, and has aromatic character,namely, a π-electron conjugated system. For example, carbon cluster C₆₀is a molecule composed of 60 carbon atoms linked to have a soccer ballshape. The carbon cluster can be prepared by subjecting graphite orcarbon to an arc discharge, resistive heating, laser beam heating,magnetron sputtering, etc. to obtain soot and purifying the soot bysolvent extraction, column chromatography and the like to a high purityof 99.9% or more.

Besides the highly purified C₆₀, low purity C₆₀, i.e., a C₆₀ /C₇₀mixture can also be used.

These carbon clusters generally manifest conductivity orsuperconductivity upon being doped with a dopant.

Examples of donors to be used as a dopant in the present inventioninclude cations of alkali metals and alkaline earth metals (e.g.,beryllium, magnesium, calcium, strontium, and barium); cations oftransition metals (e.g., titanium, chromium, manganese, iron, cobalt,nickel, and copper); NH₄ ⁺, and PH₄ ⁺ ; and cations of metal elementsbelonging to the groups IIb, IIIa, IVa, Va, etc. of the periodic table(e.g., Zn, Cd, Mg, Al, Ga, In, Tl, Ge, Sn, Pb, Sb, and Bi).

Examples of acceptors to be used as a dopant in the present inventioninclude at least one member selected from the group consisting of atrihalide ion (e.g., I₃ ⁻, Br₃ ⁻ or IBr₃ ⁻), AuI₂ ⁻, AuBr₂ ⁻, NO₃ ⁻, BF₄⁻, ClO₄ ⁻, ReO₄ ⁻, PF₆ ⁻, AsF₆ ⁻, Cl.H₂ O, Cu(NCS)₂ ⁻, and Cu[N(CN)₂]Br⁻. These acceptors may also used as a dopant.

Preferred examples of the dopant for the carbon cluster used in thestabilized carbon cluster conducting material according to the presentinvention include Group I elements, Group II elements, Group IIIelements, and Group IV elements. Among these, an alkali metal, analkaline earth metal, a noble metal, In, and Sn are more preferably usedfor the stabilized carbon cluster conducting material, and K, Rb, Cs,Ca, Ba, Sr, Au, Ag, Cu, In, and Sn are particularly preferred.

Doping of the above-mentioned dopant into a carbon cluster can beperformed by various known doping methods, such as a heating method, anelectrolysis method or a gas phase method, selected according to theshape of the carbon cluster or other conditions. The electrolysis methodincludes an electrochemical oxidation-reduction method at a constantvoltage or a constant current, with an electrochemicaloxidation-reduction method at a constant voltage being preferred. Thegas phase method includes vacuum diffusion and ion implantation.

Because of the particular structure of carbon clusters represented byC_(2n), especially the soccer ball shape of C₆₀, the carbon cluster ofthe present invention is expected to have three-dimensional electriccharacter and therefore promising for broadened applications withoutrestrictions which have been associated with conventional organicconducting or superconducting materials due to their one-dimensional ortwo-dimensional electric character.

The concentration of the dopant is not particularly limited and variesdepending on the kind of dopant, the required properties of the dopedcarbon cluster. In alkali metal-doped carbon clusters M_(x) C₆₀ where Mis an alkali metal, x is preferably from 1 to 5, and the doped carboncluster exhibits superconductivity when x is 3. In indium-doped carbonclusters In_(x) C₆₀, x is preferably from 1 to 20, and more preferablyfrom 5 to 6.

Of the doped carbon clusters of the present invention, some of alkalimetal-doped ones, for example, Rb- or Cs-doped ones, have a Tc exceeding30 K. In particular, CsRbC₆₀ shows current density of the order of 10⁶A/cm².

Carbon clusters doped with a transition metal, such as Fe or Cr, arealso expected useful as a magnetic material.

By using a transition metal element other than alkali metals andalkaline earth metals, e.g., the metal element of Group IIb, IIIa, IVaor Va of the periodic table, as a dopant, there are obtained carboncluster conducting materials or superconducting materials which are muchmore stable in air than the alkali metal- or alkaline earth metal-dopedC₆₀ conducting materials or superconducting materials.

The stabilized carbon cluster conducting material or superconductingmaterial according to the present invention comprises theabove-mentioned doped carbon cluster as a core and a sheath made ofvarious metals or non-metallic substances such as insulators and therebywithstands long-term use in air.

In particular, an Sn-doped C₆₀ superconducting material as a core iscapable of providing a superconducting wire or cable which has a Tc ashigh as 37 K and displays isotropic characteristics in a magnetic field.

Examples of the metals used as a sheath include so-called syntheticmetals, such as conductive high polymers and graphite, as well asgeneral metals, e.g., copper, silver, nickel, and stainless steel.Examples of the insulators used as a sheath include glass, quartz,ceramics, diamond, and various high polymers. Metals as a sheathmaterial not only bring about chemical stabilization of the carboncluster superconducting material but serve as stabilizing materialswhich act as a bypass for an overcurrent in case of destruction of thesuperconducting state. In particular, copper or silver as a sheathmaterial is effective for stabilization of superconductivity.

The thickness of the sheath can appropriately determined depending onthe material of the sheath and the shape of the carbon clusterconducting or superconducting material as a final product, so thatoxygen or water in the air is prevented from entering in the dopedcarbon cluster. The thickness of the sheath may be from less than 1 μmto several mm, and a further thicker sheath may be employed.

Where the final carbon cluster conducting or superconducting material isa superconducting wire, the sheath is preferably a conductor, and theratio of the sectional area of the superconducting carbon cluster coreand that of the sheath can be appropriately determined according to aconventional manner known in the art of superconducting wires.

Where the final carbon cluster conducting or superconducting material isa device, an insulator (dielectric material) is preferably used as asheath (protective film). If the device is those utilizing interactionsto magnetic fields or electromagnetic waves, e.g., a superconductivequantum interference device (SQUID) and a high frequency antenna, thethickness of the sheath is generally from less than 1 μm to several μm.

The above-mentioned stabilized carbon cluster conducting materialaccording to the present invention can be produced by thoroughly mixingand grinding a starting carbon cluster and a dopant, filling a sheathwith the mixture, and subjecting the core/sheath structure to a dopingtreatment, e.g., a heat treatment, to perform doping. Before the heattreatment, the sheath filled with the core material may be evacuated to10⁻⁶ Torr to remove the inside traps. The heat treatment for doping maybe followed by pressing or wire drawing to produce wires, cables and thelike.

The heat treatment is carried out at a temperature ranging from 400° to800° C., which is higher than the temperature having conventionally beenapplied to an alkali metal doping, i.e., about 390° C., for a period offrom about 60 hours to about 30 days.

The shape of the carbon cluster conducting or superconducting materialas a final product is not particularly limited, and examples thereofincludes wires having a circular section, flat wires, multi-core wires,twisted wires, films, sheets, fabrics, rods, plates, spheres, fineparticles, fibers, and thin films formed on a substrate to form adevice.

Since the stabilized carbon cluster conducting material of the presentinvention has a conducting or superconducting carbon cluster doped with,a dopant, e.g., an alkali metal, etc. covered with a sheath, the dopedcarbon cluster as a core is prevented from contacting with oxygen orwater in air and thereby protected from chemical denaturation.

Examples of the sectional structure of the stabilized carbon clusterconducting material are shown in FIGS. 1 through 5. In FIGS. 1 to 4,numbers 1 and 2 indicate a core and a sheath, respectively. Thestabilized carbon cluster conducting material shown in FIG. 5 comprisessubstrate 3 having formed thereon thin film core 1' which is coveredwith sheath 2'.

In addition to the excellent stability in air, the stabilized carboncluster conducting material of the present invention can be supplied ina wide variety of shapes, such as a wire, a multi-core wire, and asheet, whereas the conventional carbon cluster conducting materials orsuperconducting materials were available only in the form of powder or athin film.

Accordingly, the stabilized carbon cluster conducting materials of thepresent invention are of high industrial value as conducting orsuperconducting materials of light weights available in a variety offorms in the fields of, for example, electromagnetic shields, magneticshields, bearings, magnet, various wires, and sensors.

The device using a carbon cluster film according to the presentinvention is characterized by a structure comprising a substrate havingformed thereon a conducting or superconducting carbon cluster film, atleast the carbon cluster film being covered with a protective filmsubstantially impermeable to oxygen and water in the atmosphere.

According to this structure, the dopant in the carbon cluster film isprevented from reacting with oxygen or water in the atmosphere so thatthe characteristics of the carbon cluster film can be retained in astable manner for a prolonged period of time.

Because the carbon cluster molecule is a π-electron conjugated moleculeand thereby more reactive than inorganic substances, e.g., GaAssemiconductors, it permits modification with various functional groups,by which a variety of characteristics would be given to the carboncluster film.

The above-mentioned device of the present invention can be produced by aprocess comprising the steps of:

depositing a carbon cluster on a substrate while adding a dopant to forma carbon cluster conducting or superconducting film on a substrate; and

forming a protective film impermeable to oxygen and water in theatmosphere to cover the carbon cluster conducting or superconductingfilm.

The device of the present invention is preferably produced by a processcomprising the steps of:

depositing a carbon cluster on a substrate while adding a dopant to forma carbon cluster conducting or superconducting film on a substrate;

depositing a carbon cluster on the carbon cluster conducting orsuperconducting film to form a carbon cluster film; and

forming a protective film impermeable to oxygen and water in theatmosphere to cover the carbon cluster conducting or superconductingfilm and the carbon cluster film.

All the steps in the processes according to the present invention arepreferably carried out in a continuous vacuum line without exposing thecarbon cluster film formed on the substrate to the atmosphere.

By conducting all the steps in a continuous vacuum line, since both thecarbon cluster film and the protective film are prevented from contactwith the atmosphere, there is obtained a device with satisfactorycharacteristics. Further, a protective film formed in vacuo can have adense structure thereby effectively interferes with permeation of oxygenor water.

Preferred examples of the dopant for the carbon cluster used in thedevice according to the present invention include Group I elements,Group II elements, Group III elements, and Group IV elements. Amongthese, an alkali metal, an alkaline earth metal, a noble metal, In, andSn are more preferably used for the device, and K, Rb, Cs, Ca, Ba, Sr,Au, Ag, Cu, In, and Sn are particularly preferred.

Since the carbon cluster film is formed by simultaneous deposition ofthe carbon cluster and the dopant, the amount of the dopant can easilybe controlled simply by varying the rate of dopant deposition. As aresult, a carbon cluster film having arbitrary conductivity, inclusiveof from a nearly insulator to a semiconductor-like material, can beobtained with extreme ease and good reproducibility.

The protective film preferably comprises an insulator having acoefficient of thermal expansion close to that of the carbon clusterfilm. For example, considering that an alkali metal- or indium-dopedcarbon cluster film has a thermal expansion coefficient of 3×10⁻⁶ /°C.,an insulator having a thermal expansion coefficient ranging from 1×10⁻⁶to 5×10⁻⁶ /°C. are preferred. Examples of such an insulator includessilicon (2.6×10⁻⁶ /°C.), silicon dioxide (SiO₂) (2.9 to 3.2×10⁻⁶ /°C.),silicon nitride (Si₃ N₄) (1.3×10⁻⁶ /°C.), diamond (1.0×10⁻⁶ /°C.),amorphous carbon (1.0×10⁻⁶ /°C.), and boron nitride (1.8×10⁻⁶ /°C.). Thecloseness between these materials and the doped carbon cluster film inthermal expansion coefficient eliminates the fear of cracking in theprotective film when used in a low temperature or high temperaturecondition. That is, the protective film exhibits sufficient durabilityagainst thermal shocks enough to avoid troubles due to cracks, such aspermeation of oxygen or water into the carbon cluster film. Retention ofthe characteristics of the device can thus be further improved.

The protective film of silicon or an oxide or nitride thereof can beformed by MBE deposition, CVD in a vacuum system, plasma CVD,sputtering, etc.

The thickness of the carbon cluster film is generally from 100 to 1,000nm, and the thickness of the protective film is from 0.1 to 10 μm.

One embodiment of the process for producing a device of the presentinvention, in which an alkali metal-doped cluster film, is used isdescribed below by referring to one example shown in FIG. 6.

Substrate 1 has formed thereon gold electrode pattern 2. Carbon clustersuperconducting film 3 is formed on substrate 1 in contact with goldelectrode 2. Carbon cluster film 3 in this example is composed of acarbon cluster doped with an alkali metal (R), R_(x) C₆₀. The alkalimetal used herein includes rubidium (Rb) and potassium (K).

Carbon cluster film 3 is covered with insulating film 4 as a protectivefilm. Insulating film 4 is designed so as to prevent permeation ofoxygen and water in the atmosphere at room temperature. Specifically,insulating film 4 may have sufficient density or thickness enough toprevent permeation of oxygen or water. Further, insulating film 4 ismade of a material whose coefficient of thermal expansion is close tothat of carbon cluster film, i.e., 3×10⁻⁶ /°C., such as those enumeratedabove.

Carbon cluster film 3 being protected by insulating film 4, oxygen orwater in the air is prevented from entering therein, whereby the alkalimetal present in carbon cluster film 3 is kept away from reacting, andsatisfactory superconductivity can be retained for a prolonged period oftime.

Another embodiment of the process according to the present invention inwhich an indium-doped carbon cluster film, is described below byreferring to an illustrative example shown in FIG. 7.

Gold electrode pattern 2 is formed on substrate 1. Carbon cluster film 3is formed on substrate 1 in contact with gold electrode 2. Carboncluster film 3 in this embodiment is composed of layer 31 comprising anindium-doped carbon cluster, In_(x) C₆₀, which is formed by simultaneousdeposition of C₆₀ and indium and layer 32 solely comprising C₆₀ as anupper layer. C₆₀ layer 32 functions as a stress-absorbing layer betweenC₆₀ /In layer 31 and protective film 4 hereinafter described. In otherwords, when protective film 4 is formed thereon, there are fears thatthe device constituting protective film 4, e.g., silicon, may enter C₆₀/In layer 31 and that thermal or physical stress may destroy the surfaceof C₆₀ /In layer 31 to cause deterioration of its characteristics,leading to deterioration of the characteristics of the finally obtaineddevice. Such troubles can be eliminated by providing C₆₀ layer 32. Thethickness of C₆₀ layer 32 is generally from 100 to 50 nm.

Carbon cluster film 3 is covered with protective film 4. Protective film4 is designed so as to prevent permeation of oxygen or water in the airat room temperature. Specifically, it may have sufficient density orthickness enough to prevent permeation of oxygen or water. Further,protective film 4 of this embodiment is made of silicon whosecoefficient of thermal expansion is close to that of carbon cluster film3.

Carbon cluster film 3 being protected by protective film 4, oxygen orwater in the air is prevented from entering therein, whereby indiumcontained in carbon cluster film 3 can be kept in the condition equal tothe stage immediately after doping. As a result, satisfactorycharacteristics can be retained for a prolonged period of time.

In FIG. 8 is shown an MBE deposition apparatus which can be used for theproduction of the device according to the present invention. Substrate 1is held on susceptor 51 in vacuum chamber 50 evacuated to about 10⁻⁷ to10⁻⁹ Torr. Molecular beam source 52 for generating a molecular beam ofC₆₀, molecular beam source 53 for generating a molecular beam of adopant such as an alkali metal (e.g., rubidium) or indium, and molecularbeam source 54 for generating a molecular beam of a protectivelayer-forming material such as silicon are provided to face substrate 1.Above each molecular beam source 52, 53 or 54 is set shutter 55, 56 or57, respectively, with which generation of molecules is to becontrolled. A molecular beam source for generating a molecular beam ofC₆₀ or an alkali metal has a resistive heating element composed of aKnudesen cell, and that for generating a molecular beam of indium orsilicon has an electron bombardment heating element composed of anelectron gun.

The present invention can be applied to a Josephson junction device. InFIG. 10 is shown an example of a Josephson junction device to which thepresent invention is applied. A Josephson junction device can beproduced by interposing insulating film 30 between a pair of carboncluster superconducting films 3a and 3b. In this embodiment, insulatinglayer 30 can be formed in the same vacuum line by depositing a carboncluster film C₆₀ with no doping of an alkali metal with shutter 55 openand shutters 56 and 57 closed.

In the production of such a Josephson junction device, since formationof carbon cluster superconducting films 3a and 3b can be carried out ata temperature not higher than 300° C., insulating layer 30 is notdamaged during the formation of the upper superconducting film 3b.Accordingly, a satisfactory Josephson junction can be formed with asmall thickness of insulating layer 30 to provide a high quality device.In addition, being protected by insulating film 4, carbon clustersuperconducting films 3a and 3b maintain their superconductivity for anextended period of time.

Therefore, the Josephson junction device of FIG. 10 is useful as a SQUIDor a high speed switching device for logic operation with computers.

The present invention will now be illustrated in greater detail withreference to Examples, but it should be understood that the presentinvention is not limited thereto.

EXAMPLE 1

A commercially available C₆₀ /C₇₀ carbon cluster was purified by silicagel column chromatography using a hexane/benzene mixture (95/5 byvolume) as an eluent to obtain purified C₆₀ carbon cluster.

Cesium (Cs), rubidium (Rb) and the C₆₀ carbon cluster were weighed at amolar ratio of 2/1/1 and sealed into a glass tube for doping in a glovedbox equipped with an Ar-circulating cleaning device. The glass tube withthe contents was set in a tubular furnace, where the glass tube wasevaluated to 10⁻² Torr, re-sealed, and heated at 390° C. for 3 days. Theresulting CsRbC₆₀ was packed in a copper-made pipe in a gloved box, thepipe evacuated to 10⁻² Torr, and the both ends were pressed and sealedby soldering.

The copper pipe filled with CsRbC₆₀ was fabricated by swaging to obtaina wire having an outer diameter of about 2 mm. As a result ofmeasurement of D.C. magnetic susceptibility, superconductivity wasobserved at a Tc of 30° K. The wire underwent no change in Tc whenallowed to stand in air for one month or longer.

EXAMPLE 2

Sn powder and the purified C₆₀ cluster powder were weighed at a molarratio of 4/1, thoroughly mixed and ground in an argon atmosphere. Thegrinds were packed in a silver pipe, the pipe evacuated to 10⁻⁶ Torr,and the both ends of the pipe were pressed and sealed with silversolder. The pipe was heated at 600° C. for 10 days and rolled into aflat wire, followed by annealing at 600° C. for 10 days.

As a result of measurement of D.C. magnetic susceptibility, theresulting wire showed diamagnetism at Tc of 33 K. When allowed to standin air for 1 month or longer, the wire underwent no change in Tc.

EXAMPLE 3

Silicon substrate 1 (plane orientation: (110)) having a dimension of 5mm×5 mm with a thickness of 0.5 mm having previously formed thereon fourgold electrodes 2 as shown in FIG. 9-(a) was set in the MBE depositionapparatus shown in FIG. 8. C₆₀ powder, an Rb alloy, and Si were put inmolecular beam source 52, 53, and 54, respectively. Chamber 50 wasevacuated to a degree of vacuum of about 10⁻⁸ Torr.

Carbon cluster C₆₀ and metallic rubidium were simultaneously evaporatedfrom molecular beam sources 52 and 53, respectively, with shutters 55and 56 open to form carbon cluster superconducting film 3 composed of anRb-doped carbon cluster (Rb₃ C₆₀) on substrate 1 as shown in FIG. 9-(b).

Silicon was then evaporated from molecular beam source 54 with shutter57 open and shutters 55 and 56 closed to form insulating film 4 having adeposit thickness of 4,000 Å so that carbon cluster superconducting film3 was covered therewith as shown in FIG. 9-(c). The resulting device hada cross section as shown in FIG. 6. Each gold electrode 2 extended outof insulating film 4.

The device was taken out in the air, and electric resistance wasmeasured at a temperature of 4.2 to 100 K. with a four-terminal network.It was confirmed as a result that the device was transformed into asuperconducting state at a temperature not higher than Tc of 30 K.

The same measurement on the device after being warmed to roomtemperature and allowed to stand in the atmosphere for 1 day revealed nochange of the critical temperature.

Further, the device was subjected to a durability test against heatcycles between a liquid helium temperature and room temperature. As aresult, no change in the critical temperature was observed even after 10heat cycles. This indicates that the device suffers from no cracks evenif cooled to a cryogenic temperature and is therefore protected frompermeation of oxygen or water in the atmosphere into carbon clustersuperconducting film 3.

It is thus seen that insulating film 4 comprising silicon effectivelyfunctions as a protective film for carbon cluster superconducting film 3thereby to prevent a reduction of rubidium in superconducting film 3.That is, insulating film 4 inhibits permeation of oxygen and water inthe atmosphere to assure retention of satisfactory superconductivity foran extended period of time. Such dense insulating film 4 can first beobtained by forming carbon cluster superconducting film 3 and insulatingfilm 4 in a continuous vacuum line.

Further, the difference in coefficient of thermal expansion between Siand Rb₃ C₆₀ is extremely small. More specifically, the coefficient ofthermal expansion of Si is 2.5×10⁻⁶ /°C., and that of Rb₃ C₆₀ is 3×10⁻⁶/°C. That is, the coefficient of thermal expansion of insulating film 4is practically equal to that of carbon cluster superconducting film 3 aswell as that of substrate 1. It is understood accordingly that thedevice has sufficient durability against thermal shocks.

Furthermore, since gold electrodes 2 extend out of insulating film 4,terminals for testing electrical characteristics, etc. can be connectedto these gold electrodes 2 in the atmosphere without causingdeterioration of carbon cluster superconducting film 3.

EXAMPLE 4

A superconducting device was produced in the same manner as in Example3, except for replacing silicon used as a material for insulating film 4for protection of carbon cluster superconducting film 3 with diamond,amorphous carbon or boron nitride having a coefficient of thermalexpansion of 1.0×10⁻⁶ /°C., 1.0×10⁻⁶ /°C. or 1.8×10⁻⁶ /°C.,respectively, which is close to those of carbon cluster superconductingfilm 3 and silicon substrate 1.

Similarly to Example 3, the resulting devices each underwent nodeterioration of the Rb₃ C₆₀ film and exhibited sufficient durabilityagainst thermal shocks.

EXAMPLE 5

Quarts glass substrate 1 having a dimension of 10 mm×10 with a thicknessof 0.5 mm having previously formed thereon four gold electrodes 2 asshown in FIG. 11-(a) was set on susceptor 51 of the MBE depositionapparatus of FIG. 8. To each gold electrode 2 was connected a terminalof an electric resistance measuring apparatus according to afour-terminal network method which was placed out of the depositionapparatus. C₆₀ powder, indium, and silicon were put in molecular beamsources 52, 53, and 54, respectively.

Chamber 50 was evacuated to a degree of vacuum of 10⁻⁷ to 10⁻⁸ Torr, andcarbon cluster C₆₀ and indium were simultaneously evaporated frommolecular beam sources 52 and 53 with shutters 55 and 56 open whilemonitoring the resistivity between 4 gold electrodes to form C₆₀ /Inlayer 31 comprising an indium-doped carbon cluster, In_(x) C₆₀, having athickness of 350 nm on substrate 1 kept at room temperature. The finalresistivity of C₆₀ /In deposit layer 31 in vacuo was 2 Ω·cm, which is asemiconductor-like resistivity.

Then, shutter 56 was closed to stop the evaporation of indium frommolecular beam source 53, and evaporation of carbon cluster C₆₀ frommolecular beam source 52 was continued to deposit C₆₀ layer 32 having athickness of 200 nm on C₆₀ /In layer 31 to form carbon cluster film 3 asshown in FIG. 11-(b).

Shutter 55 was closed to stop the evaporation of carbon cluster C₆₀ frommolecular beam source 52 and, instead, shutter 57 was opened toevaporate silicon from molecular beam source 54. There was thusdeposited silicon protective film 4 to a thickness of 600 nm with whichcarbon cluster film 3 was covered as shown in FIG. 11-(c). The resultingdevice had a cross section as shown in FIG. 7. Each gold electrode 2extended out of protective film 4.

The device was taken out in the atmosphere while continuing measuringthe electric resistivity of C₆₀ /In layer 31, and the measurement wasfurther continued in the atmosphere. The results of the measurement areshown in FIG. 12 (solid line). The time was reckoned from the point atwhich the device was taken out. It is seen that the device retains aresistivity on the order of kΩ even after 9 days.

For comparison, a device was produced in the same manner as describedabove, except for forming no protective film 4. The results of theresistivity measurement in the atmosphere are also shown in FIG. 12(broken line). It is seen that the resistivity abruptly increases in theatmosphere, reaching an order of GΩ within 5 minutes.

These results prove that covering of carbon cluster film 3 withprotective film 4 effectively prevents permeation of oxygen and water ofthe atmosphere into carbon cluster film 3.

The device of Example 5 was analyzed by Auger electron spectroscopy. InFIG. 13 is shown the composition profile of the device in its thicknessdirection (corresponding to the sputtering time) as obtained by theAuger electron spectroscopy. The profile pattern proves that C₆₀ /Inlayer 31 has a composition of In₅₋₆ C₆₀.

In C₆₀ /In layer 31 of the device of Example 5, 1.4 oxygen atoms weredetected per C₆₀ molecule. In the comparative device having noprotective film 4, to the contrary, 6.2 oxygen atoms were detected perC₆₀ molecule in the C₆₀ /In layer. These results also prove obviouslythe oxygen barrier effect of protective film 4.

Temperature dependency of electrical resistance of C₆₀ /In layer 31 ofthe device of Example 5 was determined in a temperature range of from 5to 300 K. The results obtained are shown in FIG. 14. It was confirmedthat the device exhibits the character of a semiconductor, retaining aresistivity on the order of kΩ even at 5 K. The change in resistivityobserved was reversible in a temperature rise and in a temperature fall.It can be understood that protective film 4 suffered from no cracks atsuch a very low temperature around the liquid helium temperature so thatoxygen and water of the atmosphere were inhibited from permeating intocarbon cluster superconducting film 3.

EXAMPLE 6

A quarts glass substrate having a dimension of 10 mm×10 with a thicknessof 0.5 mm having previously formed thereon four gold electrodes was seton the susceptor of the MBE deposition apparatus of FIG. 8. To each ofthe gold electrode was connected a terminal of an electric resistancemeasuring apparatus according to a four-terminal network method whichwas placed out of the deposition apparatus. C₆₀ powder, an Rb alkalidispenser and silicon were put in the molecular beam sources,respectively. The molecular beam sources for C₆₀ and Rb had a resistiveheating element composed of a Knudesen cell, and that for Si had anelectron bombardment heating element composed of an electron gun.

The chamber was evacuated to a degree of vacuum of 10⁻⁷ to 10⁻⁸ Torr,and carbon cluster C₆₀ was evaporated from the molecular beam source todeposit on the substrate at 100° C. a layer composed of C₆₀ only havinga thickness of 200 nm. The evaporation of C₆₀ was then stopped, and Rbwas evaporated to deposit where the temperature of the substrate was 70°C. Upon deposition of C₆₀ and Rb, the resistivity of between the fourgold electrodes was monitored. The evaporated Rb was dispersed into theC₆₀ layer in a gas phase to accomplish doping with Rb. The compositionof the superconductive Rb-doped C₆₀ was Rb₃ C₆₀ at which composition,the resistivity showed the minimum value. Upon doping of Rb, theevaporation of Rb was stopped when the resistivity showed the minimumvalue 20 Ω.

The shutter for the Si source was opened to evaporate Si. There was thusdeposited a silicon protective film to a thickness of 500 nm with whichthe superconducting Rb₃ C₆₀ film was covered. The resulting device had across section as shown in FIG. 15. In FIG. 15, numeral 1 denotes thesubstrate, 2 denotes the gold electrode, 3 denotes the superconductingRb₃ C₆₀ film, 4 denotes the silicon protective film, and 5 denotes awiring connected to the resistance measuring apparatus.

The resulting device was analyzed by Auger electron spectroscopy. Theprofile pattern obtained proves that the superconducting layer has asuperconducting composition of Rb₃ C₆₀.

Temperature dependency of electrical resistance of the superconductinglayer of the device was determined. The results obtained are shown inFIG. 16. It was confirmed that the device exhibited transfer tosuperconducting state at Tc (on set) of 24 K and Tc (end) of 10 K.

The device was then exposed to the atmosphere for 220 minutes with theresistivity measurement being continued. The resistivity was increasedonly 1.3 times the initial value. Temperature dependence of electricalresistance of the superconducting layer of the device was againdetermined. The results obtained are shown in FIG. 17. It was confirmedthat the device still exhibited the superconducting transition at Tc (onset) of 24 K and Tc (end) of 10 K. Thus, deterioration insuperconducting properties of the device due to exposure to theatmosphere was not observed.

For comparison, a device was produced in the same manner as describedabove, except for forming no protective film. The resistivity began toabruptly increase when exposed to the atmosphere.

These results prove that covering of the carbon cluster film with theprotective film effectively prevents permeation of oxygen and water ofthe atmosphere into the carbon cluster film to improve the stability ofthe device in the atmosphere.

EXAMPLE 7

The same procedures as in Example 6 were repeated to form a devicecomposed of a substrate having thereon an Rb-doped C₆₀ film covered withan Si protective film. An Al protective film was further formed on theSi protective film by evaporating from a molecular beam source having anelectron bombardment heating element. The resulting device had a crosssection as shown in FIG. 18. In FIG. 18, numeral 1 denotes thesubstrate, 2 denotes the gold electrode, 3 denotes the superconductingRb₃ C₆₀ film, 4 denotes the silicon protective film, 5 denotes a wiringconnected to the resistance measuring apparatus, and 6 denotes the Alprotective film.

Temperature dependence of electrical resistance of the superconductinglayer of the device was determined. The results obtained are shown inFIG. 19. The device was exposed to the atmosphere for 8 hours andtemperature dependence of electrical resistance was again determined.The results of obtained are shown in FIG. 20. It was confirmed from theresults in FIGS. 19 and 20 that the device exhibited the superconductingtransition at Tc (on set) of 18 K even after exposure to the atmospherefor 8 hours.

These results prove that the provision of the Al protective film furtherimproves the stability of the device in the atmosphere.

While C₆₀ was used as a carbon cluster in the foregoing Examples, itdoes not mean exclusion of use of carbon cluster C₇₀ in place of C₆₀.

As described and demonstrated above, the device according to the presentinvention prevents a dopant, such as an alkali metal or indium in thecarbon cluster film, from reacting with oxygen or water in theatmosphere and therefore retains satisfactory conductivity orsuperconductivity for an extended period of time.

According to the process for producing the device of the presentinvention, since a carbon cluster film and a protective film can beformed in a continuous vacuum line, the carbon cluster film is neverbrought into contact with the atmosphere during the production. Inaddition, the protective film formed in vacuo is so denseness thatreliability in preventing oxygen or water from permeating into thecarbon cluster film and in retaining satisfactory superconductivity.

Further, by using an insulating material having a coefficient of thermalexpansion ranging from 1×10⁻⁶ /°C. to 5×10⁻⁶ /°C. as the above-mentionedprotective film, the durability of the protective film against thermalshocks can be improved to prevent cracking. Permeation of oxygen orwater can thus be prevented with certainty to thereby further improveretention of superconductivity. For example, Si has a coefficient ofthermal expansion of 2.6×10⁻⁶ /°C., which is very close to that ofIn_(x) C₆₀, i.g., 3×10⁻⁶ /°C. Therefore, protective film 4 comprising Sihas a coefficient of thermal expansion substantially equal to that ofcarbon cluster film 3, from which it is understood that the deviceexhibits sufficient durability against thermal shocks.

Further, since gold electrodes 2 extend out of protective film 4,terminals for, for example, testing of electrical characteristics can beconnected to these gold electrodes 2 in the atmosphere without causingdeterioration of carbon cluster superconducting film 3.

While the invention has been described in detail and with reference tospecific examples thereof, it will be apparent to one skilled in the artthat various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A device comprising a substrate having thereon,(1) a film of a conducting or superconducting carbon cluster comprising(a) C₆₀ or (b) a C₆₀ /C₇₀ mixture, each of (a) and (b) being doped withat least one dopant selected from the group consisting of Group Ielements, Group II elements, Group III elements, and Group IV elements,covered with (2) a protective film capable of substantially preventingpermeation of oxygen and water from the atmosphere,wherein saidprotective film (2) comprises an insulator having a thermal expansioncoefficient ranging from 1×10⁻⁶ /°C. to 5×10⁻⁶ /°C., wherein said devicefurther comprises (3) a non-doped carbon cluster film between said film(1) and said protective film (2), wherein said insulator is selectedfrom the group consisting of silicon, silicon dioxide, silicon nitride,diamond, amorphous carbon, and boron nitride, and wherein saidprotective film (2) has a thickness of from 0.1 μm to 10 μm.
 2. Thedevice as claimed in claim 1, wherein said dopant is selected from thegroup consisting of an alkali metal, an alkaline earth metal, a noblemetal, In, and Sn.
 3. The device as claimed in claim 2, wherein saiddopant is In.